Polyuretidione adducts of polyisocyanates are intermediates which can be used in the preparation of high performance urethane coatings, paints, and films. These adducts provide reduced volatility and an associated reduced toxicity hazard during use, as compared to monomeric polyisocyanates, such as, for example, toluene diisocyanate. In addition, because of their low viscosity, isocyanato uretidiones can be used as reactive diluents for other highly viscous or solid isocyanate group containing coatings components or as a polyisocyanate component in solvent-free and low solvent coatings formulations.
Processes for preparing these adducts are well known. Examples illustrative of these processes can be found in U.S. Pat. Nos. 4,476,054; 4,912,210; and 4,929,724. Generally, the prior art processes involve adding a soluble catalyst which promotes the isocyanate to uretidione (also known as "dimerization") reaction of the precursor isocyanate, optionally in the presence, but usually in the absence, of a solvent, allowing the reaction to proceed to the desired extent and then stopping the reaction with a suitable quenching agent which destroys the activity of the catalyst. Alternatively, in the cases where relatively volatile catalysts are used, the reaction is stopped by distilling the catalyst along with the residual, unreacted precursor isocyanate from the product dimer.
After the residual, unreacted precursor isocyanate is removed, the resulting material, in the case where the precursor isocyanate is a diisocyanate, is a mixture of oligomers composed of 2, 3, 4, etc. precursor diisocyanate molecules joined by 1, 2, 3, etc. uretidione rings. Usually, this mixture is simply called dimer,.
In the case where the precursor isocyanate is polyisocyanate, the reaction is generally stopped well before all the isocyanate groups have been converted to uretidione groups because, otherwise, the resulting product would be an unusable polymer having a very high (theoretically infinite) molecular weight and viscosity. However, the cost of equipment and energy to remove residual, unreacted precursor isocyanate dictate that the reaction not be stopped too soon. Generally, the reaction is run to more than 10% conversion but less than 50% conversion. The preferred range is between 20 and 35%. The reaction is typically stopped using a quenching agent. The reaction between conventional dimerization catalysts and quenching agents typically results in the formation of an insoluble product which is typically removed by filtration using a filter aid.
Unfortunately, both the quenching agent and the filter aid increase the likelihood of introducing undesirable impurities into the product. Accordingly, new processes for producing dimers that do not employ a quenching agent and filter aid(s), and employ fewer process steps than prior art processes, would be highly desired by the dimer manufacturing community. Alternatively, in the cases where relatively volatile catalysts are used: the catalyst is contained in the recovered precursor isocyanate, making it unsuitable for any other use except recycle to the dimerization process; and, because the dimerization reaction is thermally reversible, especially in the presence of a catalyst, some of the product dimer is converted back to precursor isocyanate before the catalyst is removed at elevated temperatures. Accordingly, new processes for producing dimers that provide higher yields of product dimer, as well as catalyst-free recovered precursor isocyanate, would also be highly desired by the dimer manufacturing community.
An approach to meeting this need would be a catalyst that is bound to an insoluble substrate. Such systems are described in German Patent DE 4,026,705 wherein trialkyl phosphines are adsorbed onto matrices with specific surface characteristics. However, this technique suffers from two deficiencies. First, trialkyl phosphines promote the formation of isocyanurates in addition to uretidiones. This results in a higher viscosity product and also makes the product unsuitable for applications wherein cross linked, thermoset, films and elastomers are not desired. Secondly, the phosphine is not chemically bound to the substrate, therefore, some portion of the catalyst remains in solution after the substrate is filtered from the reaction mixture. Consequently, the addition of a catalyst inhibitor is still required before the unreacted isocyanate precursor is removed from the final product. Of course, this product is thereby contaminated with catalyst and inhibitor residues.
A more preferable approach is disclosed for a trimerization process in co-pending U.S. patent application 07/844,265, filed on Mar. 2, 1992 wherein the catalytic site is covalently bound to an insoluble organic polymer, and this process has also been used in dimer production. The described catalyst systems overcome the above cited limitations inasmuch as it is demonstrated that no catalytic residues remain in the product after the resin is removed. Therefore, no catalyst inhibitors need to be added to the product; and, the preferred catalytic site, derived from 4-aminopyridine, exclusively promotes the formation of uretidiones, thereby giving a more desirable product, free of isocyanurates. However, organic polymer substrates have some shortcomings. It is necessary to subject the as-produced resin to a rigorous pretreatment to remove low molecular weight oligomers that would otherwise contaminate the uretidione product. Even with such a pretreatment, moderate, but undesirable, levels of color are formed in the dimerization process. Most problematic is the accumulation uretidione oligomers within the resin during the dimerization reaction. This phenomenon of oligomer fouling continuously reduces the activity of the catalyst resin and thereby limits its single use lifetime. Further, the trapped oligomers can not be easily washed from the substrate. Thus, they represent an undesirable yield loss and necessitate the use of very rigorous conditions to regenerate the catalyst resin for reuse.
While, in principle, the aforementioned problems can be reduced through the use of more highly cross linked ("macroporous" or "macroreticular") polymers, they can not be eliminated as there is still some migration of the precursor isocyanate into the resin. Further, it is difficult to prepare such polymers with practical levels of catalytic sites available on the surface of the resin bead. Thus, a most preferred approach would be embodied in systems wherein the catalytic sites are covalently bound to the surface of an inorganic matrix. In principle, all of the above described problems arising from the dissolution of the precursor isocyanate into the catalytic substrate would be circumvented. Heretofore, such isocyanate dimerization catalyst systems have not been known to the knowledge of the present inventors.